The present invention relates to bone prostheses and to methods for enhancing the anchoring of such prostheses or promoting the regrowth of bone in the vicinity of a prosthetic implant or other bone repair.
Replacement of a bone joint, or the splicing or repair of traumatic bone injury often involves the insertion or external attachment of an elongated member that spans a fracture, or that forms a joint termination for mechanical articulation with a mating part at the damaged portion of bone. Thus, for example, fractures may commonly be repaired with a bone plate which spans the break, or artificial joints such as knee or hip joints may utilize a prosthetic stem portion inserted into a long bone, that receives a corresponding prosthetic articulation portion attached to or articulated against the end of the stem portion for forming the replacement joint. In each case, the structural element of the prosthetic repair takes over a portion of the natural mechanical loading of the bone, but also requires regrowth of that bone in order for the prosthesis to become effectively attached to or incorporated in the bone.
It is known that healthy bone undergoes growth processes in which competing rates of bone resorption or wasting, and bone growth or accretion operate to maintain the requisite bone strength. Moreover, bone growth increases when the bone is subject to mechanical stress. Conversely, when a prosthetic stem or bone plate takes over the portion of the loading on a bone, and is anchored or transfers the load to a distal region, intermediate portions of bone may experience no natural loading, and tend to erode, an effect known as stress shielding. Much recent research in the area of prosthesis design has attempted to optimize the mechanical strength, e.g., the strain and bending deflection, of prosthetic elements, so as to match them to the characteristics of natural bone and assure that the requisite amount of loading continues to be transferred to surrounding bone to encourage the receiving residual bone to grow strong and bind to the prosthetic element.
At the physiological level, the mechanism whereby bone stresses result in enhanced bone growth or bone mass accretion are not fully elucidated. It has long been known that natural bone in mammals and even frogs has a piezoelectric property, and this has been attributed, for example, to the presence of hydroxy apatite in the bone itself. Experiments have shown that this active bone material is structured such that compression of the bone results in accumulation of a negative charge at the compressed surface, while tensile elongation produces charge of the opposite polarity. The property is intrinsic to the bone itself, and not to surrounding tissue or biological material, since the piezoelectric behavior has not been observed, for example, in tendon tissue or other non-boney structures, and it persists whether the bone is in vivo or ex vivo. Generation of charge has also been hypothesized to result from deformation of long chain molecules or other macro-molecules in the bone, based on the observation that removal of the organic fraction from natural bone may cause bone to either lose it piezoelectric property, or become so fragile as to render any charge undetectable.
In addition to the above observations on the electrogenic or charge-generating material present in natural bone, the presence of electrical potentials at the bone surface has been considered to promote bone healing, and several investigators have attempted, by applying currents or pulses of known shape, duration or energy, to experimentally assess the magnitude of this effect and to determine optimal regimens for enhancing post-operative healing. Conflicting results have been reported, with some investigators focusing on the desirability of a particular energy range and/or pulse duration during a limited post-operative time interval. As a matter of biophysics, it would seem apparent that a prolonged or excessively high DC potential would result in polarization shielding which is likely to interfere with natural processes governed by biologically available potentials applied to enzymatic or fluid transport mechanisms that may operate with the scale of tissue, cell and material mobility characteristic of a fracture site. In line with such expected effects, enhanced growth has been reported primarily for regimens involving relatively low frequency pulses at moderately low energies.
However, even when specific therapeutic effects appear to be demonstrated by the experimental data for a particular charge regimen, many practical problems are presented in terms of a delivery system for applying the desired charge pattern over a suitably long time interval. For example, one common approach suggests implanted or transdermal platinum electrodes, while another suggests an implanted power supply such a silver oxide battery or power source similar to that used for cardiac pacemakers, to enable the sustained application of an electric potential to the bone surface for an extended interval.
Accordingly it would be desirable to provide improved methods and devices for stimulating healing of fractures or implanted prostheses.